Lichen is in the family of epiphyte which are non self supporting AIR plants that need a structure to grow on. Lichen in itself is not bad but sets up the proper environment for algae to grow under or around it. Lichen might not be the bad guy but algae sure is because it grows into the bark and causing it to die.

Cooper sulfate painted on the Lichen is the best cure and Citrus Groves schedule it every 3 to 4 years.